As is known in the art, some important objectives in the design of power electronic converters are high efficiency and high power density (i.e., small size and mass), as well as the ability to maintain these efficiencies across wide operating ranges of output power and input voltage and/or output voltage.
As is also known, power density can be increased by increasing the switching frequency of a converter. To achieve high efficiencies at high power densities, power converters must operate using so-called “soft-switching” techniques—either a zero voltage switching (ZVS) technique and/or zero current switching (ZCS) technique, for example. In ZVS, a transistor voltage is constrained close to zero when switching on or off. In ZCS, a transistor current is constrained close to zero when switching on or off. Without soft switching, transistor switching loss prevents high efficiency from being obtained and also limits power density (owing to the need to operate at low switching frequencies).
Unfortunately, while available soft-switching power converters can achieve high efficiencies under specific operating conditions, performance tends to degrade greatly when considering requirements of operation across varying input voltage, output voltage and power levels. In particular, with conventional circuit designs and control methods, it is difficult to maintain desirable circuit waveforms (e.g., ZVS/ZCS switching, minimum conduction current, etc.) as power is reduced from maximum and as the input voltage or output voltage vary from nominal. This challenge in maintaining high efficiency is tied to both the circuit design and the control methodology.
To understand this challenge, consider some widely-used design and control techniques. Referring now to FIG. 1, one common means of controlling resonant soft-switched inverters (e.g., series, parallel, series-parallel converters, etc.) is frequency control, in which the output voltage is regulated in the face of load and voltage variations by modulating the converter switching frequency. Because of the inductive loading requirements to achieve ZVS switching, power is reduced in such converters by increasing switching frequency, exacerbating switching loss. Wide frequency operation also makes design of magnetic components and EMI filters more challenging. Moreover, depending upon resonant tank design, circulating currents in the converter may not back off with power, reducing power transfer efficiency.
Referring now to FIG. 1A, an alternative method of control that can be applied to bridge converters at fixed frequency is phase-shift control. In phase shift control techniques, the relative timing of multiple inverter legs are modulated to control power. However, conventional full-bridge resonant converters using phase shift control suffer from asymmetric current levels between the two inverter legs at the switching instants as the legs are outphased to reduce output power. The result is that the transistors in the leading inverter leg start to turn-off at large currents. Also, as outphasing is increased further the transistors in the lagging inverter leg lose ZVS turn-on capability. These factors result in extra losses and lead to lower converter efficiency at partial loads, and consequently to poor design tradeoffs.
Other fixed frequency control techniques, such as asymmetrical clamped mode control and asymmetrical pulse width control, have also been developed. However, these also lose zero voltage switching (ZVS) capability as the output power is reduced. Hence, they also do not maintain high efficiency across a wide load range.
Other more complicated designs have also been tried to solve the problem of achieving high efficiencies across varying input voltage, output voltage and power levels with limited success. There is, therefore, a need for converter designs and associated controls that can provide reduced loss when operating over wide voltage and power ranges.